Light-emitting diodes (LED) offer many advantages over conventional lighting apparatus, such as long lifetime, high efficiency, and non-toxic materials. With the development of electronic technology, light-emitting diodes are finding ever wider applications. For example, in consumer applications, LED light bulbs are showing promise as replacements for conventional white light incandescent or florescent light bulbs. Further, more and more electronic devices adopt LCD as display, and LEDs are becoming increasingly popular as a backlight source.
In LED applications, each LED load may be an LED string having multiple light-emitting diodes connected in series. A power switch may be coupled to a plurality of LED loads in parallel. Alternatively, an integrated circuit controller may be coupled to each one of a plurality of LED loads to control the current flow in each LED load separately. In order to improve the power efficiency, it is desirable for the power supply to provide the lowest power necessary to maintain a regulated output for the load. Therefore, it is desirable to minimize the dropout voltage for the power supply. A dropout voltage of a voltage regulator is the smallest possible difference between the input voltage and output voltage to maintain the power converter's intended operating range.
Some conventional approaches describe a feedback control of power conversion for a single LED string to provide dropout voltage optimization. Other conventional approaches provide a constant current regulator for multiple channels, but do not provide low dropout voltage optimization. Another conventional approach describes an efficiency optimizer that reduces an external LED power supply output voltage by injecting a current in a feedback loop to the power supply, if the LED strings need less power.